In the field of high-speed serial data communication, analysis of timing jitter has been of interest. Many concepts were formalized with a publication of the Fibre Channel Methodologies for Jitter and Signal Quality Specification (Technical Report TR-35-2004, Washington, D.C.: ANSI/INCITS, 2004). The methods described in the Fibre Channel Methodologies for Jitter and Signal Quality Specification allow analysis of timing jitter at a specific reference voltage. The reference voltage corresponds to a horizontal slice through an eye diagram at the given level. However, the methods make no attempt to analyze voltage noise.
U.S. Pat. No. 7,522,661, titled “METHOD OF PRODUCING A TWO-DIMENSIONAL PROBABILITY DENSITY FUNCTION (PDF) EYE DIAGRAM AND BIT ERROR RATE EYE ARRAYS,” incorporated herein by reference in its entirety, describes a method of first performing a voltage analysis on a waveform location that is flat (i.e., has zero slew rate) and is therefore theoretically unaffected by timing jitter. Then, a jitter analysis is performed on a high-slew-rate area in which the effects of noise are mathematically removed. Together, with additional steps described in U.S. Pat. No. 7,522,661, a statistical description of the eye at all points vertically and horizontally can be determined.
Sampling oscilloscopes are well known. When utilized to measure repeating high frequency electrical waveforms, such devices conventionally sample small sequential portions of successive waveforms. Thus, the cumulative result of this sampling technique provides a composite waveform readout representative of the subject waveforms. The method described in U.S. Pat. No. 7,522,661 is well-adapted to the strengths of a sampling oscilloscope, which can be configured to repetitively sample and store a particular location in a repeating waveform without wasting memory or other resources on the remainder of the waveform. The method of U.S. Pat. No. 7,522,661 gathers information to separate jitter from noise using only two sampling locations per pattern repetition.
In contrast, a real-time digitizing oscilloscope acquires and stores samples spaced closely-enough that the features of the waveform can be represented directly by the sequential samples. The method of U.S. Pat. No. 7,522,661 results in low efficiency on a real-time oscilloscope since most of the acquired samples are thrown out without being used. This lack of efficiency becomes more pronounced as pattern length increases. To accumulate sufficient statistics on sampling locations chosen as described in U.S. Pat. No. 7,522,661, a real-time oscilloscope has to acquire and process many real-time waveforms. In addition, the real-time oscilloscope possesses valuable information about the dynamic characteristics of the waveform in the vicinity of a target analysis point, such as the slope of the preceding edge, but the method of U.S. Pat. No. 7,522,661 fails to take advantage of this information.
Embodiments of the invention address these and other limitations in the prior art.